In the past, composite oxides which contain a plurality of metal oxides have been used as carriers for exhaust gas-purifying catalysts and catalytic promoters. Among these, composite oxides which contain ceria are being broadly used since ceria has an oxygen storage capacity (OSC) enabling it to absorb and release oxygen in accordance with the oxygen partial pressure. In particular, ceria-zirconia-based composite oxides are known to have excellent properties as catalytic promoters of automotive exhaust gas-purifying catalysts. Numerous proposals have been made relating to their compositions and structures and to methods of producing the same.
For example, PLT 1 discloses a method of production of zirconia powder in which a stabilizing agent is dissolved in solid state characterized by being comprised of a step of crushing zirconium hydroxide in water to form a slurry, a step of stirring the slurry while adding and mixing in an aqueous solution which contains one or more water soluble salts of yttrium, calcium, magnesium, cerium, aluminum, and other usually used zirconia stabilizing agents so as to make the majority of metal ions of the stabilizing agent be adsorbed at the zirconium hydroxide, a step of heating the treated slurry to raise it in temperature and make the amount of adsorption of the metal ions of the added stabilizing agent increase, then using ammonia water etc. to neutralize the slurry to thereby make the adsorbed metal ions of the stabilizing agent precipitate in the zirconium hydroxide, a step of making the unadsorbed remainder of the metal ions of the stabilizing agent precipitate as hydroxides on the zirconium hydroxide surface, and a step of separating the mixed precipitate of the obtained zirconium hydroxide from the aqueous solution, then calcining it.
Further, PLT 2 discloses a method of production of a zirconia-ceria-based composite oxide which comprises adding an alkali into an aqueous solution of a zirconium salt which contains 0.42 to 0.7 mole of sulfate radical (SO42-) per mole of zirconium cations at up to 50° C. temperature, adding a cerium salt solution into that reaction mixture, causing the obtained mixture react to form a zirconia-ceria-based complex hydroxide, then firing this zirconia-ceria-based complex hydroxide to form a composite oxide. It discloses that, in that case, the alkali which is added into the aqueous solution of the zirconium salt is added until the pH value of the mixture becomes a pH 1 to 2 in range and, further, that the alkali which is added into the mixture is preferably added until that pH value reaches a value of a pH 8 or more.
In recent years, these ceria-zirconia-based composite oxides, considering the environments of use, have been required to have an excellent oxygen storage property after performing a durability test which heats the oxides in the atmosphere at a temperature condition of 1100° C. for 5 hours. PLT 3 describes that “it is known that the oxygen storage capacity is greatly affected by the crystal phase of the zirconia-ceria complex. By forming a solid-solution phase of zirconia-ceria, the oxygen storage capacity is remarkably improved. For this reason, to obtain a high oxygen storage and release capacity, it is desired that there be a solid solution crystal phase”. Further, PLT 4 describes as the means for forming a solid solution crystal phase, “a method of production of a zirconium-containing composite oxide characterized by bringing a raw material solution which contains zirconium-containing salts and rare earth metal salts and/or alkali earth metal salts into contact with pulse combustion gas, then heat treating the result in an oxidizing atmosphere”.
Further, PLT 5 (claim 5) describes a composition having a specific surface area after 6 hours of a durability test under a temperature condition of 1100° C. of 5 to 13 m2/g and having zirconium oxide and cerium oxide as base materials, while PLT 6 (claim 1) describes a zirconium-cerium-based composite oxide having a specific surface area of 20 m2/g after a 6 hour-durability test under a temperature condition of 1100° C. for 6 hours.
PLT 7 illustrates a ceria-zirconia-based composite oxide which has inclusion of a precious metal (Pt, Pd, Rh, etc.) as a requirement, wherein a ratio of a mode pore diameter of pore distribution after a durability test which fires the oxide at a temperature condition of 1050° C. for 24 hours (heats it in the atmosphere) to a mode pore diameter before the durability test is substantially equal (see FIGS. 1 and 2) and describes the specific surface area as being 20 m2/g or more.
PLT 8 describes a ceria-zirconia-based composite oxide which, though having an unknown pore distribution mode pore diameter before heating, has a mode pore diameter of pore distribution of 50 nm to 70 nm in range after heating in the atmosphere at a temperature condition of 1000° C. for 24 hours.
PLT 9 has two types of groups of pores as a requirement. Paragraphs 0093 to 0095 describe a ceria-zirconia-based composite oxide which has a mode pore diameter after calcining at 900° C. for 4 hours (heating in the atmosphere) of about 45 nm and has a mode pore diameter after calcining at 1000° C. for 4 hours (heating in the atmosphere) of about 60 nm. However, the mode pore diameter of the fired precipitate after calcining the precipitate after autoclaving in the air at 850° C. for 2 hours (=before durability test) is not described.
PLT 10 describes a ceria-zirconia-based composite oxide showing the results of measurement of the pore volume before heat treatment (fresh) (see FIG. 1) and the results of measurement of the pore volume after firing at 1000° C. for 3 hours (see FIG. 2). Further, PLT 10 describes that pores which have diameters of 10 to 100 nm hold the active species of the catalyst, that is, the precious metal, well diffused, so a large pore volume which has diameters of 10 to 100 nm is sought and describes a cerium-zirconium-based composite oxide characterized by having a pore volume which has diameters of 10 to 100 nm of 0.25 ml/g or more and a pore volume which has diameters of 10 to 100 nm after heat treatment at 1000° C. for 3 hours of 0.2 ml/g or more.
PLT 11 describes “cerium oxide II characterized by exhibiting a pore volume of at least 0.1 cm3/g measured after firing at a 800 to 900° C. temperature.” It describes that the pore volume indicates the pore volume which corresponds to a 60 nm or less pore diameter and that to hold a high pore volume even after firing at 900° C., it is necessary to add a base to the cerium salt solution to form cerium hydroxide, then apply autoclaving it.
Further, PLT 12 describes a ceria-zirconia solid solution as an “oxide powder characterized by being comprised of ceria (cerium oxides), having a pore volume of pore diameters of 3.5 to 100 nm of 0.07 cc/g or more after firing at 600° C. for 5 hours, and having a pore volume of pore diameters of 3.5 to 100 nm after firing at 800° C. for 5 hours of 0.04 cc/g or more”. It is described that in ceria-zirconia for a catalytic promoter of an automotive exhaust gas-purifying catalyst, it is important to have a large pore volume as the diffusion space for exhaust gas, in particular it is important to have a pore volume whose diameters are 3.5 to 100 nm in range. PLT 12 as well, in the same way as PLT 1, is characterized by the point of neutralizing an acidic solution which contains cerium nitrate (III) by a base, then heating and aging it in water to 100 to 150° C. (autoclaving) to form a 3.5 to 100 nm in range pore volume.
Further, PLT 13 describes a composite oxide which contains Ce and Zr wherein a pore volume of 0.30 cc/g or more is realized after firing in an air atmosphere at 1000° C. for 5 hours. To obtain a porous composite oxide, a pore-forming agent constituted by a surfactant is used.
Further, PLT 14 shows two groups of pores after calcining at 900° C. temperature for 4 hours and describes an oxide which contains zirconium oxide and cerium oxide characterized in that the diameters of the pores of the first group concentrate at a value between 20 and 40 nm and the diameters of the pores of the second group concentrate at a value between 80 nm and 200 nm and characterized by having a pore volume of at least 1.5 ml Hg/g after calcining at 900° C. for 4 hours.